1. Field of the Invention
The present invention relates generally to a method and apparatus for driving an Electrophoretic Display (EPD).
2. Description of the Related Art
Recently, the concept of digital (or electronic) paper has been contemplated as a new display device incorporating the advantages of existing display devices and printed paper. Digital paper is a type of reflective display that has the most superior visual characteristics among display media, including high resolution, wide angle of view, and bright white background, like natural paper and ink. Digital paper may be implemented on any substrate, such as plastic, metal, and paper substrates. Digital paper is characterized in that it maintains an image even after power supply interruption and requires no backlight power supply, thereby extending the lifetime of a battery of a mobile communication terminal and easily reducing the manufacturing cost and the weight of the terminal. Digital paper is further characterized in that it can be applied to a larger display area than any other display devices because it can be implemented in a wide area in the same manner as existing paper. Digital display has also a memory function for preventing a displayed image from disappearing, even without power supply.
Digital paper may be implemented using an EPD. The EPD displays data in white or black according to a voltage applied between both ends thereof, and electrophoresis and microcapsules are applied in constructing the EPD. FIG. 1 illustrates a typical cell structure of an EPD, and also shows an operating principle of the EPD in section. The EPD may be formed by manufacturing transparent microcapsules each including black particles 40 and white particles 30 suspended in a colored fluid, combining the microcapsules with a binder 50, and then positioning the microcapsules combined with the binder 50 between upper and lower transparent electrodes 20 that are in contact with the inner sides of upper and lower substrates 10. When a positive voltage is applied, negatively charged fine ink particles are moved toward the surface of the EPD to display the color of the fine ink particles. To the contrary, when a negative voltage is applied, the negatively charged fine ink particles are moved downward, so that the color of the fluid can be observed. In this way, a text or an image is displayed.
The EPD is dependent on the electrostatic movement of particles floating in a transparent suspension. When a positive voltage is applied, positively charged white particles 30 electrostatically are moved to an electrode on an observer's side, and the moved white particles 30 reflect light. Contrarily, when a negative voltage is applied, the white particles 30 are moved to an electrode that is away from the observer, and the black particles 40 are moved to the upper portions of the capsules. Since the moved black particles 40 absorb the light, the observer observes the black color. Once the electrostatic movement has occurred at any polarity, the particles remain in their positions, even when the applied voltage is interrupted, and thus a memory device having bistability is provided. Dissimilar to this, an electrophoretic capsule using a single kind of particles is also proposed, which is formed in such a manner that a transparent polymer capsule includes white charged particles floating in a fluid that is dyed a dark color.
In driving the EPD, the same voltage must be applied to both the black particles and the white particles only for the same period of time in order to satisfy a Direct Current (DC) balancing condition and avoid an overdrive state. If this consideration is not kept, then the lifetime and display function of the EPD may be deteriorated.
However, when the same voltage is applied to the black particles and the white particles only for the same period of time in order to move the particles, an afterimage is caused by the difference in the mobility characteristics of the black and white particles.
FIG. 2 illustrates an example of a driving voltage pulse for driving a typical EPD. At the same voltage and for the same period of time, a positively charged black particle has good mobility whereas a negatively charged white particle has relatively poor mobility in comparison with the black particle. Assuming that the particle is moved for a period of time of 180 ms, when the display state of the EPD first transitions from white 210 to black 220, and then transitions from black 220 to white 230 again, the difference in the mobility characteristics of the black and white particles results in the lower reflectance of white 230 than that of white 210. This means that a user still observes previously displayed data, that is, an afterimage of previously displayed data is displayed to a user.
Therefore, there is a need for a way to drive an EPD without an afterimage of displayed data while satisfying a DC balancing condition.